Ultrafiltration (UF) and microfiltration (MF) membranes have become essential to the separation and purification in manufacture of biomolecules. Biomolecular manufacturing, regardless of its scale, generally employs one or more steps using filtration. The attractiveness of these membrane separations rests on several features including, for example, high separation power, and simplicity, requiring only the application of pressure differentials between feed and permeate. This simple and reliable one-stage filtering of the sample into two fractions makes membrane separation a valuable approach to separation and purification.
In one class of membrane separations, the species of interest is retained by the membrane, in which case the objective of the separation is typically to remove smaller contaminants, to concentrate the solution, or to effect a buffer exchange using diafiltration. In another class of membrane separations, the species of interest is that which permeates through the filter, and the objective is typically to remove larger contaminants. In MF, the retained species are generally particulates, organelles, bacteria or other microorganisms, and species that permeate are proteins, colloids, peptides, small molecules and ions. In UF the retained species are typically proteins and, in general, macromolecules, and species that permeate are peptides, ions and, in general, small molecules.
Permeation flux, also referred to as flux, is the permeation velocity of a solution through a filter. The ability to maintain a reasonably high flux is essential in the membrane separation filtration process. Low flux can result in long filtration times or require large filter assemblies, resulting in increased cost and large hold-up volumes retained in the modules and associated filter system equipment. The filtration process itself induces the creation of a highly concentrated layer of the retained species on the surface of the membrane, a phenomenon referred to as “concentration polarization,” which reduces the flux from initial membrane conditions. In the absence of counter measures, accumulation of retained particles or solutes on the surface of the membrane results in decreased flux and if not corrected the filtering process ceases to function efficiently. A conventional approach to overcoming the effects of concentration polarization in the practice of microfiltration and ultrafiltration is to operate the separation process in tangential flow filtration (TFF) mode.
TFF filters, modules and systems include devices having retentate channels formed by membranes through which the feed stream flows tangentially to the surface of the membrane. The tangential flow induces a sweeping action that reduces the thickness of the boundary layer, removes the retained species and prevents accumulation, thereby maintaining a high and stable flux. Because higher tangential velocities produce higher fluxes, the conventional practice of TFF requires the use of high velocities in the retentate channels, which in turn result in very high feed rates. These high feed rates result in low conversion, typically less than about 10 percent and often less than about five percent. Low conversion means that the bulk of the feed stream exits the module as retentate in a first pass without being materially concentrated in the retained solutes. Since many UF separations require high concentration factors, as high as about 99 percent, the retentate is typically recirculated back to the inlet of that module for further processing. This process requires recirculation loops. Systems with recirculation loops are complicated by the requirement of additional piping, storage, heat exchangers, valves, sensors and control instrumentation. Additionally, these systems are operated in batch mode resulting in undesirable effects, including subjecting the feed solution to processing conditions for long time periods often several hours.
A commercially important area for UF separations and purification is the purification of biomolecules for therapeutic drugs. Both naturally derived and genetically engineered biomolecules require multiple TFF steps to concentrate the biomolecule and to purify the biomolecule, including a process to wash the biomolecule by a process known as diafiltration. These TFF steps require custom systems to carry out batch processes that last several hours requiring large in-process tanks to hold the batch while it is being processed. These custom systems have large hold up volumes, are complicated and expensive, and have other limitations.
A commercially important area for UF separations and purification is the preparation of analytical samples (e.g., sample volumes less than about 1000 ml). The application of conventional TFF processes to sample preparation at the analytical scale is generally believed to be unpractical due to complications inherent in the use of pumps and recirculation loops. As a result, UF separations at these scales are practiced almost exclusively in a “dead-ended” mode, resulting in an inherently low flux due to concentration polarization. Centrifugal UF devices have been developed for this scale to mitigate the low flux of dead-ended UF separations. However, while these have become the dominant format for analytical scale UF, they typically require centrifuges capable of exposing the UF device to accelerations as high as 14,000 g. Furthermore, in spite of these accelerations, many separations still require long time periods, as long as one hour. Finally, the recovery of the retentate presents special difficulties in these approaches since it may be spread as a thin film over the surface of the membrane.
TFF has been the dominant method for the practice of MF and UF in manufacturing processes, whereas dead-ended centrifugal filtration has become the dominant method for processing of analytical samples. Other methods to practice MF and UF have been developed.
Dynamic membrane filtration is a class of filtration methods whereby the surface of the membrane is actively disrupted or agitated to induce mixing and reduce concentration polarization. Multiple dynamic filtration methods have been developed and commercialized, among them: Taylor vortex filtration (U.S. Pat. No. 7,425,265 to Schoendorfer; U.S. Pat. Nos. 4,670,147, 7,220,354 and 7,374,677 to Schoendorfer and McLaughlin); spinning disc filtration (SpinTek™ filtration system from SpinTek Corporation); vibrating membrane filtration (Vsep™ filters from New Logic and PallSep™ filter from Pall Corporation). These methods are effective in some applications but the filter modules and/or the systems are complex or expensive. In Taylor vortex and spinning disc devices the boundary layer at the surface of the membrane is disrupted by “active” mixing of the fluid in the retentate channel. In vibrating membrane filtration the filter modules and their holders are subjected to very high accelerations and torsional stresses and are prone to mechanical failure.
More recently single-pass TFF (“SP-TFF”) has been developed (U.S. Pat. Nos. 7,384,549, 7,682,511, 7,967,987 and 8,157,999 to de los Reyes and Mir). According to this mode of filtration, SP-TFF modules have long and thin channels, with or without internal staging, that enable high conversion in a single pass. SP-TFF has the advantages of TFF without the complexity of the recirculation loop. Because it is inherently a TFF process this technique has been rapidly adopted in bioprocessing for concentration processes. Diafiltration SP-TFF modules and processes have not yet been developed possibly because of the increased complexity of the flow distributors inside the module for the diafiltrate stream, lower productivity in comparison with conventional TFF and increased buffer consumption. In summary, SP-TFF modules and processes have found important applications in bioprocessing, but their rate of adoption has been limited due to the increased complexity of the internally staged SP-TFF modules.